1. Field of the Invention
The present invention relates to MOS active pixel sensors. More particularly, the present invention relates to a MOS active pixel sensor having a double slope light-to-output-voltage transfer gain characteristic.
2. The Prior Art
Integrated image sensors are known in the art. Such sensors have been fabricated from charge-coupled devices (CCDs) and as bipolar and MOS image sensors.
CMOS image sensors and image sensor circuitry may be organized in a manner similar to that which is disclosed in co-pending application Ser. No. 08/969,383, filed Nov. 13, 1997. Individual pixel sensors may be designed in a number of different ways. One possible pixel sensor design comprises a photodiode having its anode connected to a fixed voltage potential such as ground. The cathode of the photodiode is connectable to an amplifier. The cathode of the photodiode is also connectable to a reference potential via a reset switch so that the photodiode is reverse biased. The output of the amplifier is attached to a row-select switch, which is connected to a row select line and a column line.
The pixel sensor is first reset by turning on the reset switch. Then the reset switch is then turned off so that integration of photocurrent from the photodiode can begin. The current from the photodiode is integrated on the amplifier input node capacitance to form a voltage signal. At the appropriate time, the voltage on the row select line is raised, which activates the row-select switches in each pixel sensor in the row. This allows the amplifier to drive column line. The column line then leads down to more circuitry that will typically amplify and store the signal, and then convert the signal into digital form for inclusion in a digital pixel stream.
One problem encountered with prior-art imagers is a limitation on the dynamic range of images that can be captured by the array. Images that contain both low-light-level pixels and high-light-level pixels could be improved if the dynamic range of the imager could be increased.
In an active pixel sensor, the sensitivity of measuring charges generated by photons can be described as a charge-to-voltage gain or light-to-output-voltage transfer gain. Typically, in a prior art active pixel sensor, this gain is accounted for by two factors. A first factor is the reciprocal of the capacitance of the charge accumulation node in the sensor where photocharge accumulates to change a potential (a reciprocal capacitance represents units of volts per coulomb). A second factor is the gain of the readout amplifier, typically less than one using a source follower. Voltage dependence of the photodiode capacitance and other capacitances, and nonlinearities of the readout amplifier transistor can make the gain vary with level, so that the overall transfer curve may be somewhat nonlinear. A nonlinearity in which higher light intensities give lower gains is said to be compressive. A significant degree of compressive nonlinearity can have a beneficial effect on the signal-to-noise ratio of the image at low light levels, and can thereby enhance the usable dynamic range of the imager.
It is therefore an object of the present invention to provide a pixel sensor and an array of pixel sensors that overcome some of the shortcomings of the prior art.
A further object of the present invention is to provide a pixel sensor and an imaging array of pixel sensors that includes image level compression.
According to the present invention, a pixel sensor having built-in compression is disclosed. The pixel sensor of the present invention has a first light-to-output-voltage transfer gain up to a light accumulation threshold, and a second light-to-output-voltage transfer gain lower than the first light-to-output-voltage transfer gain after the light accumulation threshold. The pixel sensor of the present invention may be referred to herein as a double-slope active pixel sensor and has a larger dynamic range than pixel sensors without this feature.
A double-slope MOS active pixel sensor disposed on a semiconductor substrate comprises a first photodiode having a first terminal connected to a fixed potential and a second terminal. A second photodiode smaller than the first photodiode has a first terminal connected to a fixed potential and a second terminal. The first terminals of the first and second photodiodes are usually, but not necessarily, connected to the same potential such as ground.
A first semiconductor reset switch has a first terminal connected to the second terminal of the first photodiode and a second terminal connected to a first reset potential that reverse biases the first photodiode. A second semiconductor reset switch has a first terminal connected to the second terminal of the second photodiode and a second terminal connected to a second reset potential that reverse biases the second photodiode.
A first semiconductor amplifier has an input connected to the second terminal of the first photodiode and an output. A second semiconductor amplifier has an input connected to the second terminal of the second photodiode and an output. The outputs are coupled through one or two row-select switches to an output column line.
The first and second semiconductor reset switches each have a control element connected to a control circuit for selectively activating the first and second semiconductor reset switches.
In operation, the pixel sensor is first reset: the potentials at the second terminals of the first and second photodiodes are reset to the first and second reset potentials. The reset switches are then turned off, by taking their gates to a potential that establishes overflow potential barriers in their channels. Initially, high-gain conversion of integrated photocharge takes place. Later, integration of charge from the smaller photodiode dominates the pixel sensor output signal when the voltage of the second photodiode exceeds the voltage on the first photodiode. The two different light-to-voltage conversion gains, or slopes, give the pixel sensor a beneficial compressive characteristic.